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\def\,{\thinspace} The old nova DQ Herculis was observed with the Hubble Space Telescope (HST) Faint Object Spectrograph in the $\lambda\lambda$1150--2500\,\AA~range, over three orbits of the satellite, as part of the FOS team's GTO program. These observations were carried out in the ``rapid" mode, with a time resolution of 4.08~s, allowing us to search for pulsations of the UV light at the 71-s white-dwarf spin period, analogous to the low amplitude oscillations long known to be present in visible light. The integrated spectrum shows strong line emission in Ly$\alpha$, N\,V, Si\,IV/O\,IV], C\,IV and He\,II. The flux of all of these lines drops during the eclipse of the white dwarf by the late-type star, indicating that most of the emission is not nebular, geocoronal, or from an extended wind. The Si\,IV/O\,IV] complex is marginally resolved, with O\,IV] perhaps surprisingly contributing the majority of the flux. Furthermore, none of 300 simple photoionization models examined yields Si\,IV/O\,IV] as strong as that observed.
During the first and second HST orbits, we detect coherent 71-s pulsations in the continuum, with amplitudes of 9\% and 4\%, respectively. This is the first report of pulsed UV emission from DQ~Her. The amplitude is clearly time-variable, also known to be the case in visible light, as no pulsations are seen during the third orbit, with an amplitude upper limit of 2\%. Most interesting, however, is the behavior of the emission lines. During the second HST orbit, the strong Ly$\alpha$ emission is seen to pulse with a 12\% amplitude, and the phase of maximum light is displaced 0.3 later than that of the continuum pulse. No pulsations were detected in any other emission line, nor in Ly$\alpha$ during the other two orbits. The line and continuum pulsations clearly hold great promise as an aid in unraveling the complexities of this well-studied but still poorly understood system.
This work has been supported by NASA Grant NAG5-1630.
